The present invention relates to a switch, such as a circuit breaker, current-limiting device or electromagnetic contactor, which is expected to generate an arc when the current passed therethrough is interrupted, and to an arc extinguishing material capable of immediately extinguishing the arc and inhibiting a decrease in insulation resistance within and around an arc extinguishing chamber of the switch and at inner wall surfaces of the switch box.
In a switch kept applied with an overcurrent or rated current, when the contact of a moving contact element is opened from the contact of a fixed contact element, an arc is generated between the two contacts. To extinguish this arc, there is used an arc extinguishing device 8 as shown in FIG. 1-14 having insulator-(1) 1 and insulator-(2) 2 provided around a region where arc 9 is expected to generate between the moving contact 4 (not shown) of moving contact element 3 fixed movably by axis 7 and the fixed contact 5 of fixed contact element 6.
The term "contact portion" on "contact section" as used herein means a portion where the contact point 4 or 5 is located and which includes the contact point and its peripheral portion in the contact element.
The insulator (1) 1 and insulator (2) 2 of the arc extinguishing device 8 generate a thermal decomposition gas owing to the arc 9, and the thermal decomposition gas cools and extinguishes the arc 9.
Examples of such arc extinguishing devices include one employing an insulator comprising polymethylpentene, polybutylene or polymethyl methacrylate and 5 to 35 wt % of glass fiber included therein, one employing an insulator comprising an acrylic acid ester copolymer, aliphatic hydrocarbon resin, polyvinyl alcohol, polybutadiene, polyvinyl acetate, polyvinyl acetal, isoprene resin, ethylene-propylene rubber, ethylene-vinyl acetate copolymer or polyamide resin, and 5 to 30 wt % of glass fiber included therein, and one employing an insulator comprising a melamine resin containing at least two of .epsilon.-caprolactam, aluminum hydroxide and an epoxy resin.
If the width W of the insulator (2) 2 is reduced as compared to a typical one for the purpose of scaling down the arc extinguishing device, the distance between the insulator (2) 2 and the plane including the locus of an opening or closing movement of the moving contact element is shortened, with the result that the pressure of the thermal decomposition gas generated from the insulator (2) 2 by the arc rises as compared to the case of the typical insulator.
Further, if the decrease in distance between the aforesaid plane and the insulator (2) 2 causes the insulation resistance of the inner wall surfaces of the insulator (2) 2 extending along that plane to decrease, an arc current is more likely to flow in the inner wall surfaces than in the typical switch.
During generation of an arc in a switch, metal particles are scattered from the contact elements, contacts and other metal components existing adjacent the contacts in an arc extinguishing chamber and are deposited onto wall surfaces within and around the arc extinguishing chamber. A conventional switch does not take a measure for the problem of such scattered metal particles.
When the arc extinguishing device is scaled down, however, the density of the scattered metal particles adhering to the wall surfaces within the arc extinguishing chamber is increased, so that the insulation resistance of such wall surfaces is considerably lowered. Further, if the distance between the insulator (2) 2 and the aforesaid plane is shortened, the pressure of thermal decomposition gas to be generated from the insulator (2) 2 by an arc is increased to scatter the metal particles farther than in the conventional switch, so that the insulation resistance of wall surfaces existing outside the arc extinguishing chamber is also considerably lowered. Such scattered metal particles may reach and adhere to the inner wall of the switch box.
To realize a switch having the arc extinguishing device 8 miniaturized and exhibiting an improved current limiting or interrupting property, the provision of the insulator (1) covering a contact portion from which an arc will be generated or the insulator (2) disposed on opposite sides of the aforesaid plane or around the contact portion is effective. In this case, the arc extinguishing property of the insulators (1) and (2) is required to be enhanced.
Where the moving contact element or fixed contact element is reduced in cross-sectional area as compared to the conventional one for the purpose of miniaturizing the arc extinguishing device 8, the electrical resistance thereof is increased and, hence, the temperatures of the contact portion and the periphery thereof at the time when current is being applied to the switch are raised to higher temperatures than in the conventional switch. For this reason, the insulators (1) and (2) are required to have a higher heat resistance than the conventional ones.
As described above, where the width W of the insulator (2) is reduced as compared to that of the conventional one in order to miniaturize the arc extinguishing device 8, the distance between the insulator (2) and the plane including the locus of the opening or closing movement of the contact element is shortened, resulting in an increase of the pressure of thermal decomposition gas to be generated from the insulator (2) by arc. Therefore, the insulators (1) and (2) are required to have a higher pressure withstand strength than the conventional ones.
Further, if the distance between the aforesaid plane and the insulator (2) is shortened, the insulator (2) will be much more consumed by arc. Hence, the insulator (2) is required to have improved consumption-by-arc resistance, specifically to such a degree that a hole is not formed therein.
As described above, with the miniaturization of the arc extinguishing device 8, the metal scattered and deposited on wall surfaces within and around the arc extinguishing chamber causes the insulation resistance of the wall surfaces to be considerably decreased. Accordingly, it is required to insulate the metal particles to be scattered from metal components existing within the arc extinguishing chamber at the time of arc generation to prevent the decrease in the insulation resistance of the wall surfaces attributable to a metal layer formed of such deposited metal particles.